I just did this very thing this past weekend. Cooked a brisket and butt at the same time, sorta. First time cooking a brisket and knowing it would take longer to cook, it went on first on the top rack. Started at 11:30 PM Friday night. Around 6:30 AM Saturday, with an IT of around 165 - 170, I foiled the brisket, moved it to the bottom rack and put the pork butt on the top rack. I did it this way knowing that I was expecting the brisket to still cook until around 1 PM if not later and the pork butt would need to be foiled before the brisket was ready to come off. Around 11:30, I foiled the pork butt. Luckily I put it on the top rack otherwise I would have had to remove the brisket and all temp wires, pull the pork butt, wrap and reassemble everything. Around 1:30 - 2:00 PM, pulled the brisket, removed the point for burnt ends, let it cool some, wrapped it and placed into a cooler. Meanwhile, I put the butt on the bottom rack and the point cubes, in a foil pan, on the top rack to make burnt ends. 2 hours later, took off burnt ends and let butt continue since it wasn't ready to come off.
Around 5:00 PM, pork but was finished. I let it rest for about 30 minutes before pulling it. Most of it was for tomales, some was left for those that wanted it.
Dave